1　Introduction

       Soft-tissue tumors are frequently encountered by radiologist in daily clinical practice. Soft tissue arises from the mesenchyme, which will differentiate to become fat, skeletal muscles, peripheral nerves, blood vessels, and fibrous tissue^[1], and these components contribute to the classification of soft-tissue tumors. The WHO classification includes nine categories of soft-tissue tumors: adipocytic, fibroblastic/myofibroblastic, so-called fibrohistiocytic, smooth muscle, pericytic (perivascular), skeletal muscle, vascular, chondro-osseous, and those of uncertain differentiation^[2].

       We retrospectively reviewed 364 lesions between January 2009 and July 2011, and 91 lesions were confirmed histologically after core needle punctures or/and operations. 60 of 91 lesions were benign tumors or tumorlike lesions.

2　Benign Tumors

2.1　Hemangioma

       Hemangiomas are benign vascular lesions consisting of various vessels by which they can be further histologically classified. Soft-tissue hemangiomas frequently contain variable amounts of serpentine vessels, fat, smooth muscles, fibrous tissue, hemosiderin, and phleboliths^[3,4], and reactive bone changes can be seen in some cases^[5]. Hemangiomas are most commonly seen in the face and neck, followed by the trunk and extremities^[6]. They are common tumors in fancy and childhood but can occur in any age group^[7,8,9]. In our study, hemangiomas account for approximately 21% (19 of 91 lesions).

       At MR imaging, hemangiomas may be well-circumscribed or have poorly defined margins, and their signals depend on the different phases. In the proliferative phase these appear as a soft tissue mass isointense or hypointense to muscle on T1-weighted images, high signal on T2-weighted images and usual inhomogeneous enhancement pattern (Fig. 1). Flow void and enlarged vessel usually can be seen in T2-weighted images and X-ray angiography (Fig. 2). In the involuting phase, there is decrease in size of the mass with replacement by variable and increasing amounts of fat, loss of the high flow signal void and absence of enhancement^[10].

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Fig. 1  Hemangioma in the right knee joint in a 28-year-old man. 1A: Axial T1-weighted image shows a mild hypointense to muscle and ill-defined mass. 1B: Axial T2-weighted fat-suppressed image shows a hyperintense mass. 1C and 1D: Contrast-enhanced images show marked inhomogeneous enhancement.

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Fig. 2  Hemangioma in the right forearm in a 26-year-old man. 2A: Coronal T2-weighted fat-suppressed image shows a ill-defined hyperintense soft-tissue mass. 2B: With fluoroscopy, the lesion accepted positive contrast agent injection. The X-ray film shows an enlarged vessel communicated with the mass.

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Tab.1  The distribution of the histotypes and the locations

2.2　Neurilemmoma

       Neurilemmomas (schwannomas) are common benign neurogenic tumors, but are not included in the WHO classification of soft-tissue tumors. Neurilemmomas and neurofibromas belong to benign peripheral nerve sheath tumors, the both account for 10% of benign soft-tissue tumors^[7]. Neurilemmoma can appear as a well-defined smooth-bordered mass that may be aligned along the nerve, and can be eccentric to and separable from the nerve. The "split fat sign" can be observed in neurilemmomas: as the tumor enlarges, a surrounding rim of normal fat is maintained (Fig. 3). At MR imaging, Neurilemmomas are typically isointense to muscle on T1-weighted images and hyperintense to fat on T2-weighted images^[11,12]. In some cases, a "target sign", which represents two different components (termed antoni A and B areas), can be seen on T2-weighted images. Antoni A areas are more organised and are hypercellular, and are composed of spindle cells arranged in short bundles or interlacing fascicles. Antoni B regions are hypocellular, less organised and contain more myxoid, loosely arranged tissue, with a high water content^[13]. The central hypointense area (antoni A) on T2-weighted images histologically corresponds to fibrocollagenous tissue, whereas the outer hyperintense area (antoni B) corresponds to myxomatous tissue^[12] (Fig. 4).

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Fig. 3  Neurilemmoma in the right shank in a 53-year-old woman. 3A: Axial T1-weighted image shows a well-defined hypointense soft-tissue mass with "split fat sign" (arrow). 3B: On an axial T2-weighted fat-suppressed image, the mass is inhomogeneous hyperintense relative to muscle. 3C: Contrast-enhanced image demonstrates the mass marked enhancement.
Fig. 4  Neurilemmoma in the left upper arm in a 48-yeah-old woman. Coronal T2-weighted fat-suppressed image shows a hyperintense mass with "target sign" . The image depicts the outer hyperintese area (white arrow) and the central hypointense area (black arrow).

2.3　Lipoma

       Lipomas are the most common of soft-tissue tumors, The incidence of lipomas is up to 2.1 per 100 individuals^[14]. In our study, 11 of 91 lesions were confirmed as lipomas. They are usually encapsulated and composed of mature adipose tissue. Some lesions contain areas of infarction, fibrosis or hemorrhage, calcification, or ossification^[3]. Soft-tissue lipomas are categorized by anatomic location, as either superficial or deep. The superficial lipomas are common, more sharply circumscribed, and smaller than deep-seated ones. The deep-seated lipomas occur most commonly in the retroperitoneum, chest wall, and deep soft tissue of the hands and feet^[15].

       On MR images, lipomas depict typical fatty signal with homogeneous high signal on T1-weighted and low signal on STIR (Fig. 5) or T2 weighted fat-saturated images^[16]. But, in Kransdorf’s study, 31% (11of 35) of lipomas showed nonadipose content, which the author attributed to fat necrosis and associated calcification, fibrosis, inflammation, and myxoid degeneration^[17]. The important differential diagnosis for a benign lipoma includes a well-differentiated liposarcoma, which may also demonstrate a large area of fat component^[18].

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Fig. 5  Lipoma in a 60-year-old man. T1-weighted image (5A) and T2-weighted image (5B) show a well-defined hyperintense mass. 5C: Coronal STIR sequence image shows the decreased signal intensity. 5D: Coronal LAVA sequence (contrast-enhanced) image shows non-enhancement.

2.4　Fibromatosis

       Soft-tissue fibromatoses are classified as two major groups: superficial (fascial) and deep (musculoaponeurotic) lesions. Superficial fibromatoses arise from fascia or aponeuroses at palmar, plantar, penile (Peyronie disease), and knuckle pad locations^[19,20]. Palmar fibromatosis (Dupuytren disease) and plantar fibromatosis (Ledderhose disease) are most common types of the superficial fibromatoses. Deep fibromatoses, which have a fibrotic bandlike or tendonlike consistency, are also known as desmoid tumors. Desmoid tumors originate from connective tissue in muscle, fascia, or aponeuroses. Deep fibromatoses are classified according to their intraabdominal, abdominal, or extraabdominal location.

       Although soft-tissue fibromatoses are benign and have no metastatic potential, their biologic behavior is somewhat aggressive-between that of benign fibrous proliferations (eg, nodular fasciitis) and fibrosarcoma. Some researchers are of the opinion that aggressive fibromatosis (desmoid tumor) is derived from mesenchymal progenitor cells^[21].

       At MR imaging, superficial fibromatoses show predominantly isointense or hypointense to that in skeletal muscle on T1-and T2-weighted images^[22]. MR images may give some clues for the optimal timing of surgical treatment of palmar fibromatosis, given that mature collagenous lesions with relatively low signal intensity on T2-weighted images may be less likely to locally recur than are more cellular lesions with higher signal intensity on T2-weighted images^[23,24]. On MR images of deep fibromatosis, an infiltrative border in the form of a fascial tail is frequently observed. The signal intensity of fibromatosis depends on the different amounts and variable distribution of spindle-shaped cells, extracellular collagen, and myxoid matrix^[25,26]. Early-stage lesions with more cellular depict hyperintense signal on T2-weighted images. As lesions evolve, collagen deposition increases and cellularity and extracellular spaces decrease, and correspondingly the signal intensity on T2-weighted images decreases^[27] (Fig. 6).

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Fig. 6  Deep fibromatosis in the right leg in a 24-year-old man. 6A: Sagittal T2-weighted fat-suppressed image shows a ill-defined hyperintense mass. 6B: On sagittal T1-weighted image, the lesion demonstrates low signal intensity. 6C: Sagittal contrast-enhanced image shows the lesion with marked, inhomogenous enhancement.

2.5　Fibroma of the tendon sheath

       Fibroma of the tendon sheath is uncommon benign tumor of the tendon sheath, which is usually manifested as a firm, well-defined mass attached to the tendon sheath. The upper extremities, particularly the fingers, hands, and wrists, account for the site of 82% of all lesions^[28,29]. Histologically, fibroma of the tendon sheath tends toward hypocellularity, with scattered spindle-shaped myofibroblasts embedded in a dense collagenous stroma with slitlike vascular channels and cleftlike spaces^[19,30,31]. At MR imaging, the typical appearance of fibroma of the tendon sheath is attachment to a tendon or tendon sheath. The tumor shows equal to or lower than that of skeletal muscle on T1-and T2-weighted images (Fig. 7). Areas of increased signal intensity may be seen in lesions with components of increased cellularity of myxoid change. The contrast enhancement of fibroma of tendon sheath is variable. The tumor can sometimes be difficult to differentiate from other soft-tissue tumors commonly found in the extremities, such as giant-cell tumor (GCT) of the tendon sheath and pigmented villonodular synovitis (PVNS). Because of hemosiderin deposition, GCT of the tendon sheath and PVNS may show a "blooming artifact" of emphasized low signal intensity, a feature that is rarely seen in fibroma of the tendon sheath^[32].

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Fig. 7  Fibroma of the tendon sheath in a left hand in a 55-year-old man. Axial (7A) and coronal (7D) T1-weighted image show a ill-defined lesion with isointense signal to muscle. Axial (7B) and coronal (7C) T2-weighted fat-suppressed image show the leision with mild high signal intensity to the muscle. The lesion is attachment to the tendon sheath. 7E and 7F: The contrast-enhanced images show the lesion with marked enhancement.

2.6　Synovial cyst

       Synovial cysts are common juxtaarticular tumorlike lesions, which are the result of either herniation of the synvial membrane through the joint space or fluid distention of a bursa and may or may not communicate with the joint space^[33]. Popliteal cyst known as Baker cyst, which arises from the gastrocnemius-semimembranosus bursa, is the most common synovial cyst. It is usually related to joint effusion, which may be the result of joint disorders, such as meniscal tears, degenerative arthropathy, rheumatoid arthritis, pyogenic arthritis, pigmented villonodular synovitis, etc^[34]. On MR images, popliteal cysts show a well-defined fluid collection, and septa are seen in about 50% of cases^[35] (Fig. 8). The "comma sign" may be seen in some cases. A narrow neck connecting the cyst with the joint space contributes to the important sign with which we may make differential diagnosis from other lesions.

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Fig. 8  Synovial cyst in the left knee joint in a 56-year-old man. 8A and 8B: Axial T1-weighted and T2-weighted images show a cyst-like mass with linear septum. 8C: Sagittal T2-weighted fat-suppressed image shows a multiloculated cyst. 8D: On sagittal T2-weighted fat-suppressed image, a neck of the cyst is seen clearly (arrow).

2.7　Myositis ossificans

       Myositis ossificans is a benign tumorlike lesion that occurs in muscle with ossification, which is considered to be associated with trauma. Myositis ossificans at pathology has three components: a central zone of proliferating fibroblasts, a middle zone containing osteoblasts and foci immature bone, and a peripheral layer with mature bone trabeculae^[36]. Patients may be asymptmatic or may present with pain, swelling, and, occasionally, an elevated erythrocyte sedimentation rate^[37]. The MR imaging is in accord with the different stages of myositis ossificans. In the acute phase(< 2 weeks) the lesion is poorly defined and isointense in on T1-weighted images, heterogeneously T2 hyperintense, and diffuse surrounding soft-tissue edema^[38]. The lesion may also contain fluid-fluid levels due to prior to hemorrhage. In the subacute phase (3 to 6—8 weeks), peripheral foci of low signal intensity are identifiable on both T1-and T2-weighted images, owing to bone formation. In acute and subacute phases, enhancement with gadolinium is visible at the periphery of the lesion and in the adjacent edematous soft tissues. In the chronic phase (> 6—8 weeks), the lesion is well-defined and the soft-tissue edema disappears. Because of the progression of peripheral calcification and ossification (Fig. 9), more extensive low signal intensity is seen with all sequences^[39]. Peripheral ossification can be detected radiographically at 4—6 weeks, and CT is more sensitive than radiography and MR at detecting early mineralization^[40].

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Fig. 9  Myositis ossificans in the left thigh in a 25-year-old man. 9A: Axial T1-weighted image shows a well-defined lesion with slightly high signal intensity relative to muscle. 9B and 9E: Axial T2-weighted image and coronal T2-weighted fat-suppressed image show the lesion with high signal intensity. 9C and 9D: Axial CT images demonstrate the peripheral calcification or ossification. There is no obvious change in adjacent bone cortex. 9F: Coronal T1-weighted fat-suppressed contrast-enhanced image shows the lesion with peripheral enhancement.

2.8　Uncommon benign soft-tissue tumors

2.8.1　Angioleiomyoma

       Angioleiomyoma is a benign smooth muscle tumor, which can occur anywhere in the body, but is mostly often seen in the extremities. It can be located in the dermis, the subcutaneous fat, or the superficial fasciae of the extremities. Some researchers suggest angioleiomyoma is the proliferation of smooth muscle in a hemangioma, or arises from vein walls. This tumor is usually manifested as a small, slowly-growing, firm, and mobile nodule. Pain, with or without tenderness, is the predominant clinical feature of angioleiomyoma^[41]. On MR images, the tumor shows signal intensity similar to that of muscle on T1-weighted images and slightly higher signal intensity relative to muscle with a hypointense peripheral rim on T2-weighted images^[42] (Fig. 10).

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Fig. 10  Angioleiomyoma in the right foot in a 56-year-old man. 10A: Axial T1-weighted image shows a well-defined subcutaneous nodule with low signal intensity. 10B: On axial T2-weighted image, the lesion demonstrates uniform high signal intensity. 10C, 10D and 10E: The lesion depicts marked enhancement. As the scan time delays, the lesion's enhancement increases.

2.8.2　Myolipoma

       Myolipoma is a rare benign lipomatous tumor that is seen predominantly in the 5th and 6th decades of life with a female predilection^[43,44,45]. It is most commonly located in the abdominal cavity, retroperitoneum and inguinal areas. The tumor usually shows a well-defined soft-tissue mass, ranging from 10 to 25 cm in size with a median size of 17 cm. Histologically, myolipoma is composed of smooth muscle and mature adult adipose tissue. The smooth muscle component often predominates, which is usually regularly interspersed with the adipose tissue, creating a "sievelike" appearance^[46]. There is few literature about MR findings of myolipoma. On MR images, the mature lipomatous component can be well demonstrated (Fig. 11), the nonlipomatous component reveals intermediate signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted images^[47]. Calcification can be seen occasionally in large lesions.

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Fig. 11  Myolipoma in the left forearm in a 52-year-old woman. Sagittal (A) and axial (D) T1-weighted image T1 show a well-defined lipomatous mass with hypointense septa. B: Sagittal T2-weighted image shows the lesion with the decreased signal intensity. C: Contrast-enhanced image shows the lesion with linear enhancement.

2.8.3　Epidermal cyst

       Epidermal cyst is a benign cystlike mass filled with keratin debris and bounded by a wall of stratified squamous epithelium^[48]. Epidermal cyst arises from the proliferation of epidermal cell within a restrained dermal space. Epidermal cysts usually occur in the hair-bearing areas of the body, such as the scalp, face, neck, trunk and back. It can be divided into two categories, congenital and acquired epidermal cysts. At MR imaging, epidermal cysts are manifested as a well-defined round or ovoid mass with high signal intensity on T2-weighted images (Fig. 12). On T1-wwighted images, the signal intensity is variable and ranging from low signal intensity to high signal intensity. Central nonenhancement and peripheral thin rim enhancement is seen in contrast-enhangced images.

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Fig. 12  Epidermal cyst in the right hand in a 54-year-old man. 12A: Axial T2-weighted image shows a well-defined subcutaneous mass with high signal intensity. 12B: On T1-weighted image, the lesion depicts low signal intensity. 12C: Contrast-enhanced image shows the lesion with mild enhancement.

3　Conclusion

       Benign soft-tissue tumors are most common lesions in our daily radiologic practice. The MR imaging features of the soft-tissue tumors may give some indication for the diagnosis or differential diagnosis.